1. Field of the Invention
The present invention relates to an operating key switch unit having a waterproof case with an electric circuit enclosed therein, and a plurality of key switch buttons for external operation that are exposed on the front of the case.
2. Description of the Prior Art
A conventional operating key switch unit will be described with reference to FIGS. 1 to 4. FIG. 1 is a perspective view showing an appearance of the switch unit; FIG. 2 is a sectional view taken on line II--II of FIG. 1; FIG. 3 is a partially enlarged view of FIG. 2; and FIG. 4 is a front view of a switch panel. In these figures, a waterproof case 1 is molded from a resin; a display opening 2 is formed on the case front 1a of the case 1; a plurality of button holes 3 are formed (twelve in the drawings) on the case front 1a of the case 1; a printed circuit board 4 is mounted in the case 1 using bolts 5; and a display portion 6 is displaced inserted in the display opening 2 between a front-side surface 4a of the printed circuit board 4 and the case front 1a. A switch panel 7 is a panel switch inserted between the front-side surface 4a of the printed circuit board 4 and the case front 1a. Key switch buttons 8 are exposed respectively from the button holes 3 and formed integrally with the switch panel 7 using an elastic material for allowing external operation. An electroconductive contact 9 of each key switch button 8 is integral with the switch panel 7, and the electroconductive contact 9 is provided in a recess 7a. A contact 10 is printed on the front-side surface 4a of the printed circuit board 4 opposite to each electroconductive contact 9. An electronic part 11 is mounted on a back-side surface 4b of the printed circuit board 4.
The operation of the conventional key switch unit of the above construction will now be explained. The switch panel 7 is placed on the printed circuit board 4 having the electronic part 11 and the switch panel 7 is fixed to the case 1 with bolts 5. As a result, the key switch buttons 8 of the switch panel 7 are exposed from the button holes 3 of the case so that they can be operated from the exterior.
The contacts 10 on the printed circuit board 4 and the electroconductive contacts 9 of the switch panel 7 jointly form a switch structure. Normally, the contacts 9 and 10 are not in contact with each other as shown in FIG. 3, but when a key switch button 8 is depressed, the switch panel 7 is deformed through the associated recess 7a, so that the contacts 9 and 10 are in contact with each other. As a result, the contents of the operation are displayed on the display portion 6.
According to the conventional operating key switch unit described above, the panel switch 7 is integrally formed using an elastic material and the switch panel 7 is sandwiched between the front-side surface 4a of the printed circuit board 4 and the inner surface of the case front 1a. Thereby, invasion of water through the button holes 3 to the exterior of the switch panel 7 is prevented namely the interior of the case 1, through a surrounding contact portion 7b of the switch panel 7. However, the surrounding contact portion 7b of the switch panel 7 is held by a contact pressure induced by only the urging force of the bolts 5a. When the switch panel 7 is deformed in response to the depression of a key switch button 8, a change occurs in the contact pressure of the surrounding contact portion 7b, that is, a uniform contact pressure is not obtained. As a result, water enters the interior of the case 1 through the surrounding contact portion 7b, which causes the contacts 9 and 10 and the electronic part 11 to be corroded. Furthermore, if the mounting pitch of the bolts 5 is made smaller for increasing the contact pressure of the panel switch 7, the the number of bolts 5 increases which causes an increase in time that is required for the assembling operation, and an increase in cost of the unit.